Date: Sun, 23 Jan 2011 07:39:54 -0800 From: mdf@FreeBSD.org To: Lawrence Stewart <lstewart@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r217748 - head/sys/netinet/cc Message-ID: <AANLkTi=jc3VV1Sf6QsKQHCnnLVpL7%2B3K0qBgwdR60F%2Bp@mail.gmail.com> In-Reply-To: <201101231300.p0ND0PZi055936@svn.freebsd.org> References: <201101231300.p0ND0PZi055936@svn.freebsd.org>
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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@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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