015555fd4d
If d_alloc_parallel runs concurrently with __d_add, it is possible for
d_alloc_parallel to continuously retry whilst i_dir_seq has been
incremented to an odd value by __d_add:
CPU0:
__d_add
n = start_dir_add(dir);
cmpxchg(&dir->i_dir_seq, n, n + 1) == n
CPU1:
d_alloc_parallel
retry:
seq = smp_load_acquire(&parent->d_inode->i_dir_seq) & ~1;
hlist_bl_lock(b);
bit_spin_lock(0, (unsigned long *)b); // Always succeeds
CPU0:
__d_lookup_done(dentry)
hlist_bl_lock
bit_spin_lock(0, (unsigned long *)b); // Never succeeds
CPU1:
if (unlikely(parent->d_inode->i_dir_seq != seq)) {
hlist_bl_unlock(b);
goto retry;
}
Since the simple bit_spin_lock used to implement hlist_bl_lock does not
provide any fairness guarantees, then CPU1 can starve CPU0 of the lock
and prevent it from reaching end_dir_add(dir), therefore CPU1 cannot
exit its retry loop because the sequence number always has the bottom
bit set.
This patch resolves the livelock by not taking hlist_bl_lock in
d_alloc_parallel if the sequence counter is odd, since any subsequent
masked comparison with i_dir_seq will fail anyway.
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Naresh Madhusudana <naresh.madhusudana@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.