Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755474Ab3HaVYM (ORCPT ); Sat, 31 Aug 2013 17:24:12 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:38767 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753146Ab3HaVYK (ORCPT ); Sat, 31 Aug 2013 17:24:10 -0400 Date: Sat, 31 Aug 2013 22:23:56 +0100 From: Al Viro To: Linus Torvalds Cc: Waiman Long , Ingo Molnar , Benjamin Herrenschmidt , Jeff Layton , Miklos Szeredi , Ingo Molnar , Thomas Gleixner , linux-fsdevel , Linux Kernel Mailing List , Peter Zijlstra , Steven Rostedt , Andi Kleen , "Chandramouleeswaran, Aswin" , "Norton, Scott J" Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount Message-ID: <20130831212355.GN13318@ZenIV.linux.org.uk> References: <5220E56A.80603@hp.com> <5220F090.5050908@hp.com> <5220FD51.2010709@hp.com> <20130830205404.GF13318@ZenIV.linux.org.uk> <20130830214452.GH13318@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3933 Lines: 123 On Fri, Aug 30, 2013 at 03:30:14PM -0700, Linus Torvalds wrote: > On Fri, Aug 30, 2013 at 2:44 PM, Al Viro wrote: > > > > Point... Actually, I wonder if _that_ could be a solution for ->d_name.name > > printk races as well. Remember that story? You objected against taking > > spinlocks in printk, no matter how specialized and how narrow the area > > over which those are taken, but rcu_read_lock/rcu_read_unlock should be > > OK... Something like %pd expecting dentry pointer and producing dentry > > name. Sure, we still get garbage if we race with d_move(), but at least > > it's a contained garbage that way... > > Yes, that sounds quite reasonable. For printk, we'd probably want to > limit the max size and depth to something fairly small (32 bytes, max > four deep or something), and we cannot take cwd/root into account > since it can happen from interrupts, but other than that it doesn't > sound horrible. Hmm... OK, most of these suckers are actually doing just one component; we can look into 'print the ancestors as well' later, but the minimal variant would be something like this and it already covers a lot of those guys. Comments? diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt index 3e8cb73..259f8c3 100644 --- a/Documentation/printk-formats.txt +++ b/Documentation/printk-formats.txt @@ -168,6 +168,13 @@ UUID/GUID addresses: Where no additional specifiers are used the default little endian order with lower case hex characters will be printed. +dentry names: + %pd + + For printing dentry name; if we race with d_move(), the name might be + a mix of old and new ones, but it won't oops. %pd dentry is safer + equivalent of %s dentry->d_name.name we used to use. + struct va_format: %pV diff --git a/lib/vsprintf.c b/lib/vsprintf.c index 739a3636..941509e 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -26,6 +26,7 @@ #include #include #include +#include #include #include /* for PAGE_SIZE */ @@ -532,6 +533,56 @@ char *string(char *buf, char *end, const char *s, struct printf_spec spec) return buf; } +static void widen(char *buf, char *end, unsigned len, unsigned spaces) +{ + size_t size; + if (buf >= end) /* nowhere to put anything */ + return; + size = end - buf; + if (size <= spaces) { + memset(buf, ' ', size); + return; + } + if (len) { + if (len > size - spaces) + len = size - spaces; + memmove(buf + spaces, buf, len); + } + memset(buf, ' ', spaces); +} + +static noinline_for_stack +char *dentry_name(char *buf, char *end, const struct dentry *d, struct printf_spec spec) +{ + int n; + const char *s; + char *p = buf; + char c; + + rcu_read_lock(); + s = ACCESS_ONCE(d->d_name.name); + for (n = 0; n != spec.precision && (c = *s++) != 0; n++) { + if (buf < end) + *buf = c; + buf++; + } + rcu_read_unlock(); + if (n < spec.field_width) { + /* we want to pad the sucker */ + unsigned spaces = spec.field_width - n; + if (!(spec.flags & LEFT)) { + widen(p, end, n, spaces); + return buf + spaces; + } + while (spaces--) { + if (buf < end) + *buf = ' '; + ++buf; + } + } + return buf; +} + static noinline_for_stack char *symbol_string(char *buf, char *end, void *ptr, struct printf_spec spec, const char *fmt) @@ -1253,6 +1304,8 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr, spec.base = 16; return number(buf, end, (unsigned long long) *((phys_addr_t *)ptr), spec); + case 'd': + return dentry_name(buf, end, ptr, spec); } spec.flags |= SMALL; if (spec.field_width == -1) { -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/