143 lines
3.5 KiB
C++
143 lines
3.5 KiB
C++
/* sync.cc: Synchronization functions for cygwin.
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This file implements the methods for controlling the "muto" class
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which is intended to operate similarly to a mutex but attempts to
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avoid making expensive calls to the kernel.
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This file is part of Cygwin.
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This software is a copyrighted work licensed under the terms of the
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Cygwin license. Please consult the file "CYGWIN_LICENSE" for
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details. */
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#include "winsup.h"
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#include "miscfuncs.h"
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#include "sync.h"
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#include "thread.h"
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#include "cygtls.h"
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#undef WaitForSingleObject
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muto NO_COPY lock_process::locker;
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void
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muto::grab ()
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{
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tls = &_my_tls;
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}
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/* Constructor */
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muto *
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muto::init (const char *s)
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{
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char *already_exists = (char *) InterlockedExchangePointer ((PVOID *) &name,
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(PVOID) s);
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if (already_exists)
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while (!bruteforce)
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yield ();
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else
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{
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waiters = -1;
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/* Create event which is used in the fallback case when blocking is necessary */
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bruteforce = CreateEvent (&sec_none_nih, FALSE, FALSE, NULL);
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if (!bruteforce)
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api_fatal ("couldn't allocate muto '%s', %E", s);
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}
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return this;
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}
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#if 0 /* FIXME: Do we need this? mutos aren't destroyed until process exit */
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/* Destructor (racy?) */
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muto::~muto ()
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{
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while (visits)
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release ();
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HANDLE h = bruteforce;
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bruteforce = NULL;
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/* Just need to close the event handle */
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if (h)
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CloseHandle (h);
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}
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#endif
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/* Acquire the lock. Argument is the number of milliseconds to wait for
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the lock. Multiple visits from the same thread are allowed and should
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be handled correctly.
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Note: The goal here is to minimize, as much as possible, calls to the
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OS. Hence the use of InterlockedIncrement, etc., rather than (much) more
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expensive OS mutexes. */
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int
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muto::acquire (DWORD ms)
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{
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void *this_tls = &_my_tls;
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if (tls != this_tls)
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{
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/* Increment the waiters part of the class. Need to do this first to
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avoid potential races. */
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LONG was_waiting = ms ? InterlockedIncrement (&waiters) : 0;
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while (was_waiting || InterlockedExchange (&sync, 1) != 0)
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switch (WaitForSingleObject (bruteforce, ms))
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{
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case WAIT_OBJECT_0:
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was_waiting = 0;
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break;
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default:
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return 0; /* failed. */
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}
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/* Have to do it this way to avoid a race */
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if (!ms)
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InterlockedIncrement (&waiters);
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tls = this_tls; /* register this thread. */
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}
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return ++visits; /* Increment visit count. */
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}
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bool
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muto::acquired ()
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{
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return tls == &_my_tls;
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}
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/* Return the muto lock. Needs to be called once per every acquire. */
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int
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muto::release (_cygtls *this_tls)
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{
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if (tls != this_tls || !visits)
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{
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SetLastError (ERROR_NOT_OWNER); /* Didn't have the lock. */
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return 0; /* failed. */
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}
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/* FIXME: Need to check that other thread has not exited, too. */
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if (!--visits)
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{
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tls = 0; /* We were the last unlocker. */
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InterlockedExchange (&sync, 0); /* Reset trigger. */
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/* This thread had incremented waiters but had never decremented it.
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Decrement it now. If it is >= 0 then there are possibly other
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threads waiting for the lock, so trigger bruteforce. */
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if (InterlockedDecrement (&waiters) >= 0)
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SetEvent (bruteforce); /* Wake up one of the waiting threads */
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else if (*name == '!')
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{
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CloseHandle (bruteforce); /* If *name == '!' and there are no
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other waiters, then this is the
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last time this muto will ever be
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used, so close the handle. */
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#ifdef DEBUGGING
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bruteforce = NULL;
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#endif
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}
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}
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return 1; /* success. */
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}
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