liblzma: mt dec: Simplify by removing the THR_STOP state

The main thread can directly set THR_IDLE in threads_stop() which is
called when errors are detected. threads_stop() won't return the stopped
threads to the pool or free the memory pointed by thr->in anymore, but
it doesn't matter because the existing workers won't be reused after
an error. The resources will be cleaned up when threads_end() is
called (reinitializing the decoder always calls threads_end()).

Reviewed-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Thanks-to: Sam James <sam@gentoo.org>

diff --git a/src/liblzma/common/stream_decoder_mt.c b/src/liblzma/common/stream_decoder_mt.c
index 812b745df187384b2133b03651978cf1c58f792e..82962c64943a908da40164665f8e2f6786623561 100644 (file)
--- a/src/liblzma/common/stream_decoder_mt.c
+++ b/src/liblzma/common/stream_decoder_mt.c
@@ -23,15 +23,10 @@ typedef enum {
        THR_IDLE,
 
        /// Decoding is in progress.
-       /// Main thread may change this to THR_STOP or THR_EXIT.
+       /// Main thread may change this to THR_IDLE or THR_EXIT.
        /// The worker thread may change this to THR_IDLE.
        THR_RUN,
 
-       /// The main thread wants the thread to stop whatever it was doing
-       /// but not exit. Main thread may change this to THR_EXIT.
-       /// The worker thread may change this to THR_IDLE.
-       THR_STOP,
-
        /// The main thread wants the thread to exit.
        THR_EXIT,
 
@@ -346,27 +341,6 @@ worker_enable_partial_update(void *thr_ptr)
 }
 
 
-/// Things do to at THR_STOP or when finishing a Block.
-/// This is called with thr->coder->mutex locked.
-static void
-worker_stop(struct worker_thread *thr)
-{
-       // Update memory usage counters.
-       thr->coder->mem_in_use -= thr->in_size;
-       thr->in_size = 0; // thr->in was freed above.
-
-       thr->coder->mem_in_use -= thr->mem_filters;
-       thr->coder->mem_cached += thr->mem_filters;
-
-       // Put this thread to the stack of free threads.
-       thr->next = thr->coder->threads_free;
-       thr->coder->threads_free = thr;
-
-       mythread_cond_signal(&thr->coder->cond);
-       return;
-}
-
-
 static MYTHREAD_RET_TYPE
 worker_decoder(void *thr_ptr)
 {
@@ -397,17 +371,6 @@ next_loop_unlocked:
                return MYTHREAD_RET_VALUE;
        }
 
-       if (thr->state == THR_STOP) {
-               thr->state = THR_IDLE;
-               mythread_mutex_unlock(&thr->mutex);
-
-               mythread_sync(thr->coder->mutex) {
-                       worker_stop(thr);
-               }
-
-               goto next_loop_lock;
-       }
-
        assert(thr->state == THR_RUN);
 
        // Update progress info for get_progress().
@@ -510,7 +473,22 @@ next_loop_unlocked:
                                && thr->coder->thread_error == LZMA_OK)
                        thr->coder->thread_error = ret;
 
-               worker_stop(thr);
+               // Return the worker thread to the stack of available
+               // threads.
+               {
+                       // Update memory usage counters.
+                       thr->coder->mem_in_use -= thr->in_size;
+                       thr->in_size = 0; // thr->in was freed above.
+
+                       thr->coder->mem_in_use -= thr->mem_filters;
+                       thr->coder->mem_cached += thr->mem_filters;
+
+                       // Put this thread to the stack of free threads.
+                       thr->next = thr->coder->threads_free;
+                       thr->coder->threads_free = thr;
+               }
+
+               mythread_cond_signal(&thr->coder->cond);
        }
 
        goto next_loop_lock;
@@ -544,17 +522,22 @@ threads_end(struct lzma_stream_coder *coder, const lzma_allocator *allocator)
 }
 
 
+/// Tell worker threads to stop without doing any cleaning up.
+/// The clean up will be done when threads_exit() is called;
+/// it's not possible to reuse the threads after threads_stop().
+///
+/// This is called before returning an unrecoverable error code
+/// to the application. It would be waste of processor time
+/// to keep the threads running in such a situation.
 static void
 threads_stop(struct lzma_stream_coder *coder)
 {
        for (uint32_t i = 0; i < coder->threads_initialized; ++i) {
+               // The threads that are in the THR_RUN state will stop
+               // when they check the state the next time. There's no
+               // need to signal coder->threads[i].cond.
                mythread_sync(coder->threads[i].mutex) {
-                       // The state must be changed conditionally because
-                       // THR_IDLE -> THR_STOP is not a valid state change.
-                       if (coder->threads[i].state != THR_IDLE) {
-                               coder->threads[i].state = THR_STOP;
-                               mythread_cond_signal(&coder->threads[i].cond);
-                       }
+                       coder->threads[i].state = THR_IDLE;
                }
        }
 
@@ -1941,7 +1924,7 @@ stream_decoder_mt_init(lzma_next_coder *next, const lzma_allocator *allocator,
        // accounting from scratch, too. Changes in filter and block sizes may
        // affect number of threads.
        //
-       // FIXME? Reusing should be easy but unlike the single-threaded
+       // Reusing threads doesn't seem worth it. Unlike the single-threaded
        // decoder, with some types of input file combinations reusing
        // could leave quite a lot of memory allocated but unused (first
        // file could allocate a lot, the next files could use fewer