Re: [PATCH backport] membarrier: Explicitly sync remote cores when SYNC_CORE is requested

[Greg KH]

On Mon, Dec 14, 2020 at 10:00:43AM -0800, Andy Lutomirski wrote:
> commit 758c9373d84168dc7d039cf85a0e920046b17b41 upstream
> 
> membarrier() does not explicitly sync_core() remote CPUs; instead, it
> relies on the assumption that an IPI will result in a core sync.  On x86,
> this may be true in practice, but it's not architecturally reliable.  In
> particular, the SDM and APM do not appear to guarantee that interrupt
> delivery is serializing.  While IRET does serialize, IPI return can
> schedule, thereby switching to another task in the same mm that was
> sleeping in a syscall.  The new task could then SYSRET back to usermode
> without ever executing IRET.
> 
> Make this more robust by explicitly calling sync_core_before_usermode()
> on remote cores.  (This also helps people who search the kernel tree for
> instances of sync_core() and sync_core_before_usermode() -- one might be
> surprised that the core membarrier code doesn't currently show up in a
> such a search.)
> 
> Fixes: 70216e18e519 ("membarrier: Provide core serializing command, *_SYNC_CORE")
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/r/776b448d5f7bd6b12690707f5ed67bcda7f1d427.1607058304.git.luto@xxxxxxxxxx
> ---
> 
> My stable membarrier series depends on commit 2a36ab717e8f
> ("rseq/membarrier: Add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ").  I don't
> think it makes much sense to backport that feature, so here's a backport of
> the patch that doesn't need it.

Now queued up to 5.4.y and 5.9.y, thanks.

greg k-h