Messages in this thread | | | Subject | Re: [PATCH] [3/11] SYSCTL: Add proc_rcu_string to manage sysctls using rcu strings | From | (Eric W. Biederman) | Date | Mon, 21 Dec 2009 19:00:44 -0800 |
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"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
> On Mon, Dec 21, 2009 at 02:20:24AM +0100, Andi Kleen wrote: >> >> Add a helper to use the new rcu strings for managing access >> to text sysctls. Conversions will be in follow-on patches. >> >> An alternative would be to use seqlocks here, but RCU seemed >> cleaner. >> >> Signed-off-by: Andi Kleen <ak@linux.intel.com> > > Using the below as an example of my concern about access_rcu_string(), FYI. > >> --- >> include/linux/sysctl.h | 2 + >> kernel/sysctl.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++++ >> kernel/sysctl_check.c | 1 >> 3 files changed, 69 insertions(+) >> >> Index: linux-2.6.33-rc1-ak/include/linux/sysctl.h >> =================================================================== >> --- linux-2.6.33-rc1-ak.orig/include/linux/sysctl.h >> +++ linux-2.6.33-rc1-ak/include/linux/sysctl.h >> @@ -969,6 +969,8 @@ typedef int proc_handler (struct ctl_tab >> >> extern int proc_dostring(struct ctl_table *, int, >> void __user *, size_t *, loff_t *); >> +extern int proc_rcu_string(struct ctl_table *, int, >> + void __user *, size_t *, loff_t *); >> extern int proc_dointvec(struct ctl_table *, int, >> void __user *, size_t *, loff_t *); >> extern int proc_dointvec_minmax(struct ctl_table *, int, >> Index: linux-2.6.33-rc1-ak/kernel/sysctl.c >> =================================================================== >> --- linux-2.6.33-rc1-ak.orig/kernel/sysctl.c >> +++ linux-2.6.33-rc1-ak/kernel/sysctl.c >> @@ -50,6 +50,7 @@ >> #include <linux/ftrace.h> >> #include <linux/slow-work.h> >> #include <linux/perf_event.h> >> +#include <linux/rcustring.h> >> >> #include <asm/uaccess.h> >> #include <asm/processor.h> >> @@ -2016,6 +2017,60 @@ static int _proc_do_string(void* data, i >> } >> >> /** >> + * proc_rcu_string - sysctl string with rcu protection >> + * @table: the sysctl table >> + * @write: %TRUE if this is a write to the sysctl file >> + * @buffer: the user buffer >> + * @lenp: the size of the user buffer >> + * @ppos: file position >> + * >> + * Handle a string sysctl similar to proc_dostring. >> + * The main difference is that the data pointer in the table >> + * points to a pointer to a string. The string should be initially >> + * pointing to a statically allocated (as a C object, not on the heap) >> + * default. When it is replaced old uses will be protected by >> + * RCU. The reader should use rcu_read_lock()/unlock() or >> + * access_rcu_string(). >> + */ >> +int proc_rcu_string(struct ctl_table *table, int write, >> + void __user *buffer, size_t *lenp, loff_t *ppos) >> +{ >> + int ret; >> + >> + if (write) { >> + /* protect writers against each other */ >> + static DEFINE_MUTEX(rcu_string_mutex); >> + char *old; >> + char *new; >> + >> + new = alloc_rcu_string(table->maxlen, GFP_KERNEL); >> + if (!new) >> + return -ENOMEM; >> + mutex_lock(&rcu_string_mutex); >> + old = *(char **)(table->data); >> + strcpy(new, old); >> + ret = _proc_do_string(new, table->maxlen, write, buffer, lenp, ppos); >> + rcu_assign_pointer(*(char **)(table->data), new); >> + /* >> + * For the first initialization allow constant strings. >> + */ >> + if (!kernel_address((unsigned long)old)) >> + free_rcu_string(old); >> + mutex_unlock(&rcu_string_mutex); >> + } else { >> + char *str; >> + >> + str = access_rcu_string(*(char **)(table->data), table->maxlen, >> + GFP_KERNEL); > > So the above statement picks up table->data, then some other CPU comes > in and executes the "write" side of this "if" statement, we get > preempted before access_rcu_string() enters its RCU read-side critical > section, the grace period elapse, we resume, and ... ouch! > > One trick would be to make access_rcu_string() be a macro that does > first access to its first argument in an RCU read-side critical section. > Alternatively, pass in the address of the pointer, rather than the > pointer itself. > > Or explain to me how I am confused.
That sounds correct to me. There is also the missing rcu_dereference.
Which is less important but it would make clear that access_rcu_string does the dereference outside of the rcu critical section.
Eric
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