4.19-stable review patch. If anyone has any objections, please let me know.
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From: Ryusuke Konishi konishi.ryusuke@gmail.com
commit 936184eadd82906992ff1f5ab3aada70cce44cee upstream.
A potential and reproducible race issue has been identified where nilfs_segctor_sync() would block even after the log writer thread writes a checkpoint, unless there is an interrupt or other trigger to resume log writing.
This turned out to be because, depending on the execution timing of the log writer thread running in parallel, the log writer thread may skip responding to nilfs_segctor_sync(), which causes a call to schedule() waiting for completion within nilfs_segctor_sync() to lose the opportunity to wake up.
The reason why waking up the task waiting in nilfs_segctor_sync() may be skipped is that updating the request generation issued using a shared sequence counter and adding an wait queue entry to the request wait queue to the log writer, are not done atomically. There is a possibility that log writing and request completion notification by nilfs_segctor_wakeup() may occur between the two operations, and in that case, the wait queue entry is not yet visible to nilfs_segctor_wakeup() and the wake-up of nilfs_segctor_sync() will be carried over until the next request occurs.
Fix this issue by performing these two operations simultaneously within the lock section of sc_state_lock. Also, following the memory barrier guidelines for event waiting loops, move the call to set_current_state() in the same location into the event waiting loop to ensure that a memory barrier is inserted just before the event condition determination.
Link: https://lkml.kernel.org/r/20240520132621.4054-3-konishi.ryusuke@gmail.com Fixes: 9ff05123e3bf ("nilfs2: segment constructor") Signed-off-by: Ryusuke Konishi konishi.ryusuke@gmail.com Tested-by: Ryusuke Konishi konishi.ryusuke@gmail.com Cc: stable@vger.kernel.org Cc: "Bai, Shuangpeng" sjb7183@psu.edu Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/nilfs2/segment.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-)
--- a/fs/nilfs2/segment.c +++ b/fs/nilfs2/segment.c @@ -2207,19 +2207,28 @@ static int nilfs_segctor_sync(struct nil struct nilfs_segctor_wait_request wait_req; int err = 0;
- spin_lock(&sci->sc_state_lock); init_wait(&wait_req.wq); wait_req.err = 0; atomic_set(&wait_req.done, 0); + init_waitqueue_entry(&wait_req.wq, current); + + /* + * To prevent a race issue where completion notifications from the + * log writer thread are missed, increment the request sequence count + * "sc_seq_request" and insert a wait queue entry using the current + * sequence number into the "sc_wait_request" queue at the same time + * within the lock section of "sc_state_lock". + */ + spin_lock(&sci->sc_state_lock); wait_req.seq = ++sci->sc_seq_request; + add_wait_queue(&sci->sc_wait_request, &wait_req.wq); spin_unlock(&sci->sc_state_lock);
- init_waitqueue_entry(&wait_req.wq, current); - add_wait_queue(&sci->sc_wait_request, &wait_req.wq); - set_current_state(TASK_INTERRUPTIBLE); wake_up(&sci->sc_wait_daemon);
for (;;) { + set_current_state(TASK_INTERRUPTIBLE); + if (atomic_read(&wait_req.done)) { err = wait_req.err; break;