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Cygwin: FIFO: keep a writer count in shared memory

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When a reader opens, it needs to block if there are no writers open
(unless is is opened with O_NONBLOCK).  This is easy for the first
reader to test, since it can just wait for a writer to signal that it
is open (via the write_ready event).  But when a second reader wants
to open, all writers might have closed.

To check this, use a new '_nwriters' member of struct fifo_shmem_t,
which keeps track of the number of open writers.  This should be more
reliable than the previous method.

Add nwriters_lock to control access to shmem->_nwriters, and remove
reader_opening_lock, which is no longer needed.

Previously only readers had access to the shared memory, but now
writers access it too so that they can increment _nwriters during
open/dup/fork/exec and decrement it during close.

Add an optional 'only_open' argument to create_shmem for use by
writers, which only open the shared memory rather than first trying to
create it.  Since writers don't need to access the shared memory until
they have successfully connected to a pipe instance, they can safely
assume that a reader has already created the shared memory.

For debugging purposes, change create_shmem to return 1 instead of 0
when a reader successfully opens the shared memory after finding that
it had already been created.

Remove check_write_ready_evt, write_ready_ok_evt, and
check_write_ready(), which are no longer needed.

When opening a writer and looping to try to get a connection, recheck
read_ready at the top of the loop since the number of readers might
have changed.

To slightly speed up the process of opening the first reader, take
ownership immediately rather than waiting for the fifo_reader_thread
to handle it.
This commit is contained in:
Ken Brown
2020-07-11 14:05:23 -04:00
parent da9fea0759
commit 8ca713d70a
2 changed files with 124 additions and 166 deletions
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27
winsup/cygwin/fhandler.h
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@@ -1323,12 +1323,14 @@ struct fifo_reader_id_t
}
};
/* Info needed by all readers of a FIFO, stored in named shared memory. */
/* Info needed by all fhandlers for a given FIFO, stored in named
shared memory. This is mostly for readers, but writers need access
in order to update the count of open writers. */
class fifo_shmem_t
{
LONG _nreaders;
LONG _nreaders, _nwriters;
fifo_reader_id_t _owner, _prev_owner, _pending_owner;
af_unix_spinlock_t _owner_lock, _reading_lock, _reader_opening_lock, _nreaders_lock;
af_unix_spinlock_t _owner_lock, _reading_lock, _nreaders_lock, _nwriters_lock;
/* Info about shared memory block used for temporary storage of the
owner's fc_handler list. */
@@ -1339,6 +1341,9 @@ public:
int nreaders () const { return (int) _nreaders; }
int inc_nreaders () { return (int) InterlockedIncrement (&_nreaders); }
int dec_nreaders () { return (int) InterlockedDecrement (&_nreaders); }
int nwriters () const { return (int) _nwriters; }
int inc_nwriters () { return (int) InterlockedIncrement (&_nwriters); }
int dec_nwriters () { return (int) InterlockedDecrement (&_nwriters); }
fifo_reader_id_t get_owner () const { return _owner; }
void set_owner (fifo_reader_id_t fr_id) { _owner = fr_id; }
@@ -1351,10 +1356,10 @@ public:
void owner_unlock () { _owner_lock.unlock (); }
void reading_lock () { _reading_lock.lock (); }
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 (); }
void nwriters_lock () { _nwriters_lock.lock (); }
void nwriters_unlock () { _nwriters_lock.unlock (); }
int get_shared_nhandlers () const { return (int) _sh_nhandlers; }
void set_shared_nhandlers (int n) { InterlockedExchange (&_sh_nhandlers, n); }
@@ -1380,8 +1385,6 @@ class fhandler_fifo: public fhandler_base
HANDLE owner_needed_evt; /* The owner is closing. */
HANDLE owner_found_evt; /* A new owner has taken over. */
HANDLE update_needed_evt; /* shared_fc_handler needs updating. */
HANDLE check_write_ready_evt; /* write_ready needs to be checked. */
HANDLE write_ready_ok_evt; /* check_write_ready is done. */
/* Handles to non-shared events needed for fifo_reader_threads. */
HANDLE cancel_evt; /* Signal thread to terminate. */
@@ -1417,7 +1420,7 @@ class fhandler_fifo: public fhandler_base
void record_connection (fifo_client_handler&,
fifo_client_connect_state = fc_connected);
int create_shmem ();
int create_shmem (bool only_open = false);
int reopen_shmem ();
int create_shared_fc_handler ();
int reopen_shared_fc_handler ();
@@ -1426,8 +1429,13 @@ class fhandler_fifo: public fhandler_base
int nreaders () const { return shmem->nreaders (); }
int inc_nreaders () { return shmem->inc_nreaders (); }
int dec_nreaders () { return shmem->dec_nreaders (); }
int nwriters () const { return shmem->nwriters (); }
int inc_nwriters () { return shmem->inc_nwriters (); }
int dec_nwriters () { return shmem->dec_nwriters (); }
void nreaders_lock () { shmem->nreaders_lock (); }
void nreaders_unlock () { shmem->nreaders_unlock (); }
void nwriters_lock () { shmem->nwriters_lock (); }
void nwriters_unlock () { shmem->nwriters_unlock (); }
fifo_reader_id_t get_prev_owner () const { return shmem->get_prev_owner (); }
void set_prev_owner (fifo_reader_id_t fr_id)
@@ -1462,9 +1470,6 @@ class fhandler_fifo: public fhandler_base
{ return shmem->shared_fc_handler_updated (); }
void shared_fc_handler_updated (bool val)
{ shmem->shared_fc_handler_updated (val); }
void check_write_ready ();
void reader_opening_lock () { shmem->reader_opening_lock (); }
void reader_opening_unlock () { shmem->reader_opening_unlock (); }
public:
fhandler_fifo ();