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Cygwin: FIFO: fix problems finding new owner

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When the owning reader closes and there are still readers open, the
owner needs to wait for a new owner to be found before closing its
fifo_client handlers.  This involves a loop in which dec_nreaders is
called at the beginning and inc_nreaders is called at the end.  Any
other reader that tries to access shmem->_nreaders during this loop
will therefore get an inaccurate answer.

Fix this by adding an nreaders method and using it instead of
dec_nreaders and inc_nreaders.  Also add nreaders_lock to control
access to the shmem->_nreaders.

Make various other changes to improve the reliability of finding a new
owner.
This commit is contained in:
Ken Brown
2020-07-15 09:46:42 -04:00
parent 0ee972d1b0
commit da9fea0759
2 changed files with 63 additions and 35 deletions
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8
winsup/cygwin/fhandler.h
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@@ -1328,7 +1328,7 @@ class fifo_shmem_t
{
LONG _nreaders;
fifo_reader_id_t _owner, _prev_owner, _pending_owner;
af_unix_spinlock_t _owner_lock, _reading_lock, _reader_opening_lock;
af_unix_spinlock_t _owner_lock, _reading_lock, _reader_opening_lock, _nreaders_lock;
/* Info about shared memory block used for temporary storage of the
owner's fc_handler list. */
@@ -1336,6 +1336,7 @@ class fifo_shmem_t
_sh_fc_handler_updated;
public:
int nreaders () const { return (int) _nreaders; }
int inc_nreaders () { return (int) InterlockedIncrement (&_nreaders); }
int dec_nreaders () { return (int) InterlockedDecrement (&_nreaders); }
@@ -1352,6 +1353,8 @@ public:
void reading_unlock () { _reading_lock.unlock (); }
void reader_opening_lock () { _reader_opening_lock.lock (); }
void reader_opening_unlock () { _reader_opening_lock.unlock (); }
void nreaders_lock () { _nreaders_lock.lock (); }
void nreaders_unlock () { _nreaders_lock.unlock (); }
int get_shared_nhandlers () const { return (int) _sh_nhandlers; }
void set_shared_nhandlers (int n) { InterlockedExchange (&_sh_nhandlers, n); }
@@ -1420,8 +1423,11 @@ class fhandler_fifo: public fhandler_base
int reopen_shared_fc_handler ();
int remap_shared_fc_handler (size_t);
int nreaders () const { return shmem->nreaders (); }
int inc_nreaders () { return shmem->inc_nreaders (); }
int dec_nreaders () { return shmem->dec_nreaders (); }
void nreaders_lock () { shmem->nreaders_lock (); }
void nreaders_unlock () { shmem->nreaders_unlock (); }
fifo_reader_id_t get_prev_owner () const { return shmem->get_prev_owner (); }
void set_prev_owner (fifo_reader_id_t fr_id)