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| author |   Sebastian Andrzej Siewior <bigeasy@linutronix.de> | 2022-07-27 13:49:02 +0200 |
|---|---|---|
| committer |   Al Viro <viro@zeniv.linux.org.uk> | 2022-07-30 00:35:51 -0400 |
| commit | cf634d540a29018e8d69ab1befb7e08182bc6594 (patch) | |
| tree | 0639c640ff8089a405323e96f33ae9cc7d21e4c3 /fs/dcache.c | |
| parent | d_add_ci(): make sure we don't miss d_lookup_done() (diff) | |
| download | wireguard-linux-cf634d540a29018e8d69ab1befb7e08182bc6594.tar.xz wireguard-linux-cf634d540a29018e8d69ab1befb7e08182bc6594.zip | |
fs/dcache: Disable preemption on i_dir_seq write side on PREEMPT_RT
i_dir_seq is a sequence counter with a lock which is represented by the
lowest bit. The writer atomically updates the counter which ensures that it
can be modified by only one writer at a time. This requires preemption to
be disabled across the write side critical section.
On !PREEMPT_RT kernels this is implicit by the caller acquiring
dentry::lock. On PREEMPT_RT kernels spin_lock() does not disable preemption
which means that a preempting writer or reader would live lock. It's
therefore required to disable preemption explicitly.
An alternative solution would be to replace i_dir_seq with a seqlock_t for
PREEMPT_RT, but that comes with its own set of problems due to arbitrary
lock nesting. A pure sequence count with an associated spinlock is not
possible because the locks held by the caller are not necessarily related.
As the critical section is small, disabling preemption is a sensible
solution.
Reported-by: Oleg.Karfich@wago.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20220613140712.77932-2-bigeasy@linutronix.de
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'fs/dcache.c')
| -rw-r--r-- | fs/dcache.c | 12 |
1 files changed, 11 insertions, 1 deletions
diff --git a/fs/dcache.c b/fs/dcache.c index 94b737311b61..71969505e82e 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -2564,7 +2564,15 @@ EXPORT_SYMBOL(d_rehash); static inline unsigned start_dir_add(struct inode *dir) { - + /* + * The caller holds a spinlock (dentry::d_lock). On !PREEMPT_RT + * kernels spin_lock() implicitly disables preemption, but not on + * PREEMPT_RT. So for RT it has to be done explicitly to protect + * the sequence count write side critical section against a reader + * or another writer preempting, which would result in a live lock. + */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_disable(); for (;;) { unsigned n = dir->i_dir_seq; if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n) @@ -2576,6 +2584,8 @@ static inline unsigned start_dir_add(struct inode *dir) static inline void end_dir_add(struct inode *dir, unsigned n) { smp_store_release(&dir->i_dir_seq, n + 2); + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_enable(); } static void d_wait_lookup(struct dentry *dentry) |