sched: Prevent balance_push() on remote runqueues

[linux-2.6-microblaze.git] / fs / pipe.c

diff --git a/fs/pipe.c b/fs/pipe.c
index bfd946a..8e6ef62 100644 (file)
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -31,6 +31,21 @@
 
 #include "internal.h"
 
+/*
+ * New pipe buffers will be restricted to this size while the user is exceeding
+ * their pipe buffer quota. The general pipe use case needs at least two
+ * buffers: one for data yet to be read, and one for new data. If this is less
+ * than two, then a write to a non-empty pipe may block even if the pipe is not
+ * full. This can occur with GNU make jobserver or similar uses of pipes as
+ * semaphores: multiple processes may be waiting to write tokens back to the
+ * pipe before reading tokens: https://lore.kernel.org/lkml/1628086770.5rn8p04n6j.none@localhost/.
+ *
+ * Users can reduce their pipe buffers with F_SETPIPE_SZ below this at their
+ * own risk, namely: pipe writes to non-full pipes may block until the pipe is
+ * emptied.
+ */
+#define PIPE_MIN_DEF_BUFFERS 2
+
 /*
  * The max size that a non-root user is allowed to grow the pipe. Can
  * be set by root in /proc/sys/fs/pipe-max-size
@@ -429,20 +444,20 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 #endif
 
        /*
-        * Only wake up if the pipe started out empty, since
-        * otherwise there should be no readers waiting.
+        * Epoll nonsensically wants a wakeup whether the pipe
+        * was already empty or not.
         *
         * If it wasn't empty we try to merge new data into
         * the last buffer.
         *
         * That naturally merges small writes, but it also
-        * page-aligs the rest of the writes for large writes
+        * page-aligns the rest of the writes for large writes
         * spanning multiple pages.
         */
        head = pipe->head;
-       was_empty = pipe_empty(head, pipe->tail);
+       was_empty = true;
        chars = total_len & (PAGE_SIZE-1);
-       if (chars && !was_empty) {
+       if (chars && !pipe_empty(head, pipe->tail)) {
                unsigned int mask = pipe->ring_size - 1;
                struct pipe_buffer *buf = &pipe->bufs[(head - 1) & mask];
                int offset = buf->offset + buf->len;
@@ -781,8 +796,8 @@ struct pipe_inode_info *alloc_pipe_info(void)
        user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
 
        if (too_many_pipe_buffers_soft(user_bufs) && pipe_is_unprivileged_user()) {
-               user_bufs = account_pipe_buffers(user, pipe_bufs, 1);
-               pipe_bufs = 1;
+               user_bufs = account_pipe_buffers(user, pipe_bufs, PIPE_MIN_DEF_BUFFERS);
+               pipe_bufs = PIPE_MIN_DEF_BUFFERS;
        }
 
        if (too_many_pipe_buffers_hard(user_bufs) && pipe_is_unprivileged_user())