diff --git a/headers/build/os/support/Locker.h b/headers/build/os/support/Locker.h index cb42edb5ed..99f6ff4035 100644 --- a/headers/build/os/support/Locker.h +++ b/headers/build/os/support/Locker.h @@ -1 +1,50 @@ -#include <../os/support/Locker.h> +/* + * Copyright 2001-2010 Haiku, Inc. All rights reserved. + * Distributed under the terms of the MIT License. + */ +#ifndef _LOCKER_H +#define _LOCKER_H + + +#include + + +class BLocker { +public: + BLocker(); + BLocker(const char* name); + BLocker(bool benaphoreStyle); + BLocker(const char* name, bool benaphoreStyle); + virtual ~BLocker(); + + status_t InitCheck() const; + + bool Lock(); + status_t LockWithTimeout(bigtime_t timeout); + void Unlock(); + + thread_id LockingThread() const; + bool IsLocked() const; + int32 CountLocks() const; + int32 CountLockRequests() const; + sem_id Sem() const; + +private: + BLocker(const char* name, bool benaphoreStyle, + bool _ignored); + BLocker(const BLocker&); + BLocker& operator=(const BLocker&); + void InitLocker(const char* name, + bool benaphoreStyle); + bool AcquireLock(bigtime_t timeout, status_t* error); + + int32 fBenaphoreCount; + sem_id fSemaphoreID; + thread_id fLockOwner; + int32 fRecursiveCount; + + int32 _reserved[4]; +}; + + +#endif // _LOCKER_H diff --git a/src/build/libbe/support/Locker.cpp b/src/build/libbe/support/Locker.cpp new file mode 100644 index 0000000000..6b42d90a90 --- /dev/null +++ b/src/build/libbe/support/Locker.cpp @@ -0,0 +1,298 @@ +/* + * Copyright 2001-2009 Haiku, Inc. All rights reserved. + * Distributed under the terms of the MIT license. + * + * Authors: + * Erik Jaesler, erik@cgsoftware.com + */ + + +/*! Semaphore-type class for thread safety */ + + +#include +#include +#include + +#include + +#include "support_kit_config.h" + + +// Data Member Documentation: +// +// The "fBenaphoreCount" member is set to 1 if the BLocker style is +// semaphore. If the style is benaphore, it is initialized to 0 and +// is incremented atomically when it is acquired, decremented when it +// is released. By setting the benaphore count to 1 when the style is +// semaphore, the benaphore effectively becomes a semaphore. I was able +// to determine this is what Be's implementation does by testing the +// result of the CountLockRequests() member. +// +// The "fSemaphoreID" member holds the sem_id returned from create_sem() +// when the BLocker is constructed. It is used to acquire and release +// the lock regardless of the lock style (semaphore or benaphore). +// +// The "fLockOwner" member holds the thread_id of the thread which +// currently holds the lock. If no thread holds the lock, it is set to +// B_ERROR. +// +// The "fRecursiveCount" member holds a count of the number of times the +// thread holding the lock has acquired the lock without a matching unlock. +// It is basically the number of times the thread must call Unlock() before +// the lock can be acquired by a different thread. +// + + +BLocker::BLocker() +{ + InitLocker(NULL, true); +} + + +BLocker::BLocker(const char *name) +{ + InitLocker(name, true); +} + + +BLocker::BLocker(bool benaphoreStyle) +{ + InitLocker(NULL, benaphoreStyle); +} + + +BLocker::BLocker(const char *name, bool benaphoreStyle) +{ + InitLocker(name, benaphoreStyle); +} + + +/*! This constructor is not documented. The final argument is ignored for + now. In Be's headers, its called "for_IPC". DO NOT USE THIS + CONSTRUCTOR! +*/ +BLocker::BLocker(const char *name, bool benaphoreStyle, + bool) +{ + InitLocker(name, benaphoreStyle); +} + + +BLocker::~BLocker() +{ + delete_sem(fSemaphoreID); +} + + +status_t +BLocker::InitCheck() const +{ + return fSemaphoreID >= 0 ? B_OK : fSemaphoreID; +} + + +bool +BLocker::Lock() +{ + status_t result; + return AcquireLock(B_INFINITE_TIMEOUT, &result); +} + + +status_t +BLocker::LockWithTimeout(bigtime_t timeout) +{ + status_t result; + + AcquireLock(timeout, &result); + return result; +} + + +void +BLocker::Unlock() +{ + // The Be Book explicitly allows any thread, not just the lock owner, to + // unlock. This is bad practice, but we must allow it for compatibility + // reasons. We can at least warn the developer that something is probably + // wrong. + if (!IsLocked()) { + fprintf(stderr, "Unlocking BLocker with sem %" B_PRId32 + " from wrong thread %" B_PRId32 ", current holder %" B_PRId32 + " (see issue #6400).\n", fSemaphoreID, find_thread(NULL), + fLockOwner); + } + + // Decrement the number of outstanding locks this thread holds + // on this BLocker. + fRecursiveCount--; + + // If the recursive count is now at 0, that means the BLocker has + // been released by the thread. + if (fRecursiveCount == 0) { + // The BLocker is no longer owned by any thread. + fLockOwner = B_ERROR; + + // Decrement the benaphore count and store the undecremented + // value in oldBenaphoreCount. + int32 oldBenaphoreCount = atomic_add(&fBenaphoreCount, -1); + + // If the oldBenaphoreCount is greater than 1, then there is + // at least one thread waiting for the lock in the case of a + // benaphore. + if (oldBenaphoreCount > 1) { + // Since there are threads waiting for the lock, it must + // be released. Note, the old benaphore count will always be + // greater than 1 for a semaphore so the release is always done. + release_sem(fSemaphoreID); + } + } +} + + +thread_id +BLocker::LockingThread() const +{ + return fLockOwner; +} + + +bool +BLocker::IsLocked() const +{ + // This member returns true if the calling thread holds the lock. + // The easiest way to determine this is to compare the result of + // find_thread() to the fLockOwner. + return find_thread(NULL) == fLockOwner; +} + + +int32 +BLocker::CountLocks() const +{ + return fRecursiveCount; +} + + +int32 +BLocker::CountLockRequests() const +{ + return fBenaphoreCount; +} + + +sem_id +BLocker::Sem() const +{ + return fSemaphoreID; +} + + +void +BLocker::InitLocker(const char *name, bool benaphore) +{ + if (name == NULL) + name = "some BLocker"; + + if (benaphore && !BLOCKER_ALWAYS_SEMAPHORE_STYLE) { + // Because this is a benaphore, initialize the benaphore count and + // create the semaphore. Because this is a benaphore, the semaphore + // count starts at 0 (ie acquired). + fBenaphoreCount = 0; + fSemaphoreID = create_sem(0, name); + } else { + // Because this is a semaphore, initialize the benaphore count to -1 + // and create the semaphore. Because this is semaphore style, the + // semaphore count starts at 1 so that one thread can acquire it and + // the next thread to acquire it will block. + fBenaphoreCount = 1; + fSemaphoreID = create_sem(1, name); + } + + // The lock is currently not acquired so there is no owner. + fLockOwner = B_ERROR; + + // The lock is currently not acquired so the recursive count is zero. + fRecursiveCount = 0; +} + + +bool +BLocker::AcquireLock(bigtime_t timeout, status_t *error) +{ + // By default, return no error. + status_t status = B_OK; + + // Only try to acquire the lock if the thread doesn't already own it. + if (!IsLocked()) { + // Increment the benaphore count and test to see if it was already + // greater than 0. If it is greater than 0, then some thread already has + // the benaphore or the style is a semaphore. Either way, we need to + // acquire the semaphore in this case. + int32 oldBenaphoreCount = atomic_add(&fBenaphoreCount, 1); + if (oldBenaphoreCount > 0) { + do { + status = acquire_sem_etc(fSemaphoreID, 1, B_RELATIVE_TIMEOUT, + timeout); + } while (status == B_INTERRUPTED); + + // Note, if the lock here does time out, the benaphore count + // is not decremented. By doing this, the benaphore count will + // never go back to zero. This means that the locking essentially + // changes to semaphore style if this was a benaphore. + // + // Doing the decrement of the benaphore count when the acquisition + // fails is a risky thing to do. If you decrement the counter at + // the same time the thread which holds the benaphore does an + // Unlock(), there is serious risk of a race condition. + // + // If the Unlock() sees a positive count and releases the semaphore + // and then the timed out thread decrements the count to 0, there + // is no one to take the semaphore. The next two threads will be + // able to acquire the benaphore at the same time! The first will + // increment the counter and acquire the lock. The second will + // acquire the semaphore and therefore the lock. Not good. + // + // This has been discussed on the becodetalk mailing list and + // Trey from Be had this to say: + // + // I looked at the LockWithTimeout() code, and it does not have + // _this_ (ie the race condition) problem. It circumvents it by + // NOT doing the atomic_add(&count, -1) if the semaphore + // acquisition fails. This means that if a + // BLocker::LockWithTimeout() times out, all other Lock*() attempts + // turn into guaranteed semaphore grabs, _with_ the overhead of a + // (now) useless atomic_add(). + // + // Given Trey's comments, it looks like Be took the same approach + // I did. The output of CountLockRequests() of Be's implementation + // confirms Trey's comments also. + // + // Finally some thoughts for the future with this code: + // - If 2^31 timeouts occur on a 32-bit machine (ie today), + // the benaphore count will wrap to a negative number. This + // would have unknown consequences on the ability of the BLocker + // to continue to function. + // + } + } + + // If the lock has successfully been acquired. + if (status == B_OK) { + // Set the lock owner to this thread and increment the recursive count + // by one. The recursive count is incremented because one more Unlock() + // is now required to release the lock (ie, 0 => 1, 1 => 2 etc). + if (fLockOwner < 0) { + fLockOwner = find_thread(NULL); + fRecursiveCount = 1; + } else + fRecursiveCount++; + } + + if (error != NULL) + *error = status; + + // Return true if the lock has been acquired. + return (status == B_OK); +}