Messages in this thread | | | Date | Mon, 21 Dec 2009 18:51:31 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] [3/11] SYSCTL: Add proc_rcu_string to manage sysctls using rcu strings |
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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.
> + if (!str) > + return -ENOMEM; > + ret = _proc_do_string(str, table->maxlen, write, buffer, lenp, ppos); > + kfree(str); > + } > + return ret; > +} > + > +/** > * proc_dostring - read a string sysctl > * @table: the sysctl table > * @write: %TRUE if this is a write to the sysctl file > @@ -2030,6 +2085,10 @@ static int _proc_do_string(void* data, i > * and a newline '\n' is added. It is truncated if the buffer is > * not large enough. > * > + * WARNING: this should be only used for read only strings > + * or when you have a wrapper with special locking. Otherwise > + * use proc_rcu_string to avoid races with the consumer. > + * > * Returns 0 on success. > */ > int proc_dostring(struct ctl_table *table, int write, > @@ -2614,6 +2673,12 @@ int proc_dostring(struct ctl_table *tabl > return -ENOSYS; > } > > +int proc_rcu_string(struct ctl_table *table, int write, > + void __user *buffer, size_t *lenp, loff_t *ppos) > +{ > + return -ENOSYS; > +} > + > int proc_dointvec(struct ctl_table *table, int write, > void __user *buffer, size_t *lenp, loff_t *ppos) > { > @@ -2670,6 +2735,7 @@ EXPORT_SYMBOL(proc_dointvec_minmax); > EXPORT_SYMBOL(proc_dointvec_userhz_jiffies); > EXPORT_SYMBOL(proc_dointvec_ms_jiffies); > EXPORT_SYMBOL(proc_dostring); > +EXPORT_SYMBOL(proc_rcu_string); > EXPORT_SYMBOL(proc_doulongvec_minmax); > EXPORT_SYMBOL(proc_doulongvec_ms_jiffies_minmax); > EXPORT_SYMBOL(register_sysctl_table); > Index: linux-2.6.33-rc1-ak/kernel/sysctl_check.c > =================================================================== > --- linux-2.6.33-rc1-ak.orig/kernel/sysctl_check.c > +++ linux-2.6.33-rc1-ak/kernel/sysctl_check.c > @@ -131,6 +131,7 @@ int sysctl_check_table(struct nsproxy *n > set_fail(&fail, table, "Directory with extra2"); > } else { > if ((table->proc_handler == proc_dostring) || > + (table->proc_handler == proc_rcu_string) || > (table->proc_handler == proc_dointvec) || > (table->proc_handler == proc_dointvec_minmax) || > (table->proc_handler == proc_dointvec_jiffies) ||
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