svn commit: r217748 - head/sys/netinet/cc
mdf at FreeBSD.org
mdf at FreeBSD.org
Sun Jan 23 15:39:56 UTC 2011
For sbuf use for a sysctl you can use sbuf_init_for_sysctl() which
will, instead of growing, push the current data out using SYSCTL_OUT
to a wired user buffer. There's a few examples in the vm/ code. This
can sometimes significantly simplify the code since there's no need to
worry about held mutex/rwlock anymore.
Thanks,
matthew
On Sun, Jan 23, 2011 at 5:00 AM, Lawrence Stewart <lstewart at freebsd.org> wrote:
> Author: lstewart
> Date: Sun Jan 23 13:00:25 2011
> New Revision: 217748
> URL: http://svn.freebsd.org/changeset/base/217748
>
> Log:
> An sbuf configured with SBUF_AUTOEXTEND will call malloc with M_WAITOK when a
> write to the buffer causes it to overflow. We therefore can't hold the CC list
> rwlock over a call to sbuf_printf() for an sbuf configured with SBUF_AUTOEXTEND.
>
> Switch to a fixed length sbuf which should be of sufficient size except in the
> very unlikely event that the sysctl is being processed as one or more new
> algorithms are loaded. If that happens, we accept the race and may fail the
> sysctl gracefully if there is insufficient room to print the names of all the
> algorithms.
>
> This should address a WITNESS warning and the potential panic that would occur
> if the sbuf call to malloc did sleep whilst holding the CC list rwlock.
>
> Sponsored by: FreeBSD Foundation
> Reported by: Nick Hibma
> Reviewed by: bz
> MFC after: 3 weeks
> X-MFC with: r215166
>
> Modified:
> head/sys/netinet/cc/cc.c
>
> Modified: head/sys/netinet/cc/cc.c
> ==============================================================================
> --- head/sys/netinet/cc/cc.c Sun Jan 23 12:44:17 2011 (r217747)
> +++ head/sys/netinet/cc/cc.c Sun Jan 23 13:00:25 2011 (r217748)
> @@ -128,20 +128,37 @@ cc_list_available(SYSCTL_HANDLER_ARGS)
> {
> struct cc_algo *algo;
> struct sbuf *s;
> - int err, first;
> + int err, first, nalgos;
>
> - err = 0;
> + err = nalgos = 0;
> first = 1;
> - s = sbuf_new(NULL, NULL, TCP_CA_NAME_MAX, SBUF_AUTOEXTEND);
> +
> + CC_LIST_RLOCK();
> + STAILQ_FOREACH(algo, &cc_list, entries) {
> + nalgos++;
> + }
> + CC_LIST_RUNLOCK();
> +
> + s = sbuf_new(NULL, NULL, nalgos * TCP_CA_NAME_MAX, SBUF_FIXEDLEN);
>
> if (s == NULL)
> return (ENOMEM);
>
> + /*
> + * It is theoretically possible for the CC list to have grown in size
> + * since the call to sbuf_new() and therefore for the sbuf to be too
> + * small. If this were to happen (incredibly unlikely), the sbuf will
> + * reach an overflow condition, sbuf_printf() will return an error and
> + * the sysctl will fail gracefully.
> + */
> CC_LIST_RLOCK();
> STAILQ_FOREACH(algo, &cc_list, entries) {
> err = sbuf_printf(s, first ? "%s" : ", %s", algo->name);
> - if (err)
> + if (err) {
> + /* Sbuf overflow condition. */
> + err = EOVERFLOW;
> break;
> + }
> first = 0;
> }
> CC_LIST_RUNLOCK();
>
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