* Rusty Russell ([EMAIL PROTECTED]) wrote: > On Fri, 2007-08-24 at 11:45 -0400, Mathieu Desnoyers wrote: > > * Rusty Russell ([EMAIL PROTECTED]) wrote: > > > On Mon, 2007-08-20 at 16:26 -0400, Mathieu Desnoyers wrote: > > > > plain text document attachment (module.c-sort-module-list.patch) > > > > A race that appears both in /proc/modules and in kallsyms: if, between > > > > the > > > > seq file reads, the process is put to sleep and at this moment a module > > > > is > > > > or removed from the module list, the listing will skip an amount of > > > > modules/symbols corresponding to the amount of elements present in the > > > > unloaded > > > > module, but at the current position in the list if the iteration is > > > > located > > > > after the removed module. > > > > > > > > The cleanest way I found to deal with this problem is to sort the > > > > module list. > > > > We can then keep the old struct module * as the old iterator, knowing > > > > the it may > > > > be removed between the seq file reads, but we only use it as "get > > > > next". If it > > > > is not present in the module list, the next pointer will be used. > > > > > > > > By doing this, removing a given module will now only fuzz the output > > > > related to > > > > this specific module, not any random module anymore. Since modprobe uses > > > > /proc/modules, it might be important to make sure multiple concurrent > > > > running > > > > modprobes won't interfere with each other. > > > > > > You've reduced, but not eliminated, the problem. A new module inserted > > > is quite likely to reuse the same address. > > > > > > > Hi Rusty, > > > > Please tell me if I'm wrong, but I think it would not be a problem: > > > > - seq_read() makes sure that a buffer large enough is available so that > > m_show() can fully extract and print the information relative to 1 > > module. > > - m_start() and m_stop() takes the module_mutex, therefore within one > > seq_read(), once m_start has returned, the struct module * that we > > have is valid and will be consistent during the whole seq_read > > operation. > > - If a module is removed, and then a different one is inserted at the > > same address, while we are between two seq_reads for this given struct > > module address, the seq_reads will copy to user-space the information > > that is still in the buffer for the _first_ struct module encountered, > > not the new one. > > - After that, iteration will continue to the new struct module address, > > effectively skipping the newly inserted module. > > Indeed, I thought that this was a general problem: the seq_list code was > never intended to work on modifiable lists unless you get them in one > big read. > > If we accept this problem, what do we do about all the other users? >
Hum, I guess it would be best for them to switch to the proposed seq sorted list too. I think that having one example (module.c) that shows well how this works will be an incentive for other developers to port their seq_file code to the sorted list (I am thinking, among others, about kallsyms). Mathieu > Rusty. > -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
